Electronic circuits and components are ubiquitous in almost every field of the modern world, from communications to computing, transportation, energy production, storage and control, consumer goods, radios, navigation, and so forth. In each of these fields there continues to be a demand to produce ever smaller and less expensive products, and this demand in turn manifests as a push for ever more integrated circuits and components.
The earliest electronic products were, of course, wired by hand. While some products are still produced at least in part by hand, hand produced circuit boards have largely been supplanted by machine printed circuit boards, and discrete components have largely been supplanted by highly integrated components such as LSICs and VLICs.
Despite these advancements, most electronic devices are still generally manufactured by printing a circuit board, populating the circuit board with components, and then wave soldering or otherwise coupling the connector pins of the components to the board. There are a few devices in which portions of both circuit and components are more or less “printed” using ordinary printing techniques such as silk screening, and perhaps the most widely known examples are battery testers employed on AA and other small consumer batteries. An exemplary description is found in U.S. Pat. No. 4,702,563 to Parker (October 1987). However, such devices are severely limited as to the type and complexity of the circuit and components, and there remains a considerable need for more versatile apparatus and techniques for printing circuits and components.
This situation results at least in part from production of circuit boards by processes that are incapable of providing the multiple layers needed for many electronic components. For example, in a typical photolithography production of a circuit board, a metal film such as copper is applied to a rigid, non-conductive substrate, such as fiberglass. A sheet of the conductive metal is then laminated to the substrate, and a photoresist coated onto the laminate. The board is exposed to a light pattern using a light mask to reproduce the metal pattern desired, and exposure is followed by photoresist development. Finally, the entire board is immersed into a bath so that the applied metal left unprotected by the photoresist can be etched away. In another photolithographic process, it is known to form electrically conductive metal pathways by coating a substrate with a composition containing a reducible metal complex. In one such process, a substrate is coated with a sorbitol copper formate solution containing a photoactivated reducing agent. Upon exposure to ultraviolet radiation, unmasked areas are reduced to copper metal, and the remainder of the board is washed clean.
While such techniques are capable of producing single layers of high quality and definition, they are not well suited for depositing multiple layers, especially multiple layers of different materials. Especially inconsistent with standard circuit board printing techniques are printing of multilayer components such as batteries, which require specialized materials for the anode, cathode and electrolyte layers.
Silk Screening
Other techniques commonly used to print circuit boards, such as silk screening, are capable of printing multiple layers, but they do not lend themselves to printing anything but the simplest electronic components. For example, in U.S. Pat. No. 5,148,355 to Lowe (September 1992), the disclosure of which is incorporated herein by reference, circuits and electronic components such as conductors, resistors, capacitors, and insulators are produced by successive screen printing of polymer based conductive and resistive inks. It is claimed that Lowe's process may be employed to deposit as many as 30 to 50 layers on top of one another, with thickness control variance of 5 microns, and minimum spacing between adjacent conductors on the order of about 0.01 inch.
While advantageous in many ways, processes according to Lowe suffer from the use of inks in which metals are suspended as discreet particles, preferably silver flakes. Even though such particles may measure only 3-50 microns in diameter, such inks dry to form traces and layers in which current is carried by particle—particle contact rather than by a continuous mass. In addition to forming relatively poor conductors, the particle size effectively limits the smallest trace to several times the diameter of the largest particles. Still further, the potential for discontinuities in narrow traces and thin deposits appears to pose a problem of sufficient significance that repeated depositions, at slight offsets or angles are necessary to achieve reliable traces.
Another disadvantage is that Lowe is limited to printing electronic components that include only conductive, resistive and insulative portions. Lowe offers no guidance, for cop example, in printing batteries where the electrodes may advantageously comprise metal oxides, and the electrolyte may comprise a liquid or polymer. Lowe also offers no guidance in the printing of semiconductors.
Electroless Deposition
Even if known silk screening techniques were capable of printing the fineness required for electronic components, they would still be too slow for mass production of circuit/circuit component combinations. For such purposes higher speed printing techniques such as offset and intaglio printing are desired. U.S. Pat. No. 5,127,330 to Okazaki et al. (July 1992) discloses specialized inks that can be employed to produce very fine traces (3 μm wide and 2 μm thick) in intaglio printing, but there is no teaching or suggestion of employing the specialized inks for producing the multiple layers. Some work has been done on high-speed electroless deposition printing of electronic components, and representative disclosures of such processes are discussed below.
In U.S. Pat. No. 5,403,649 to Morgan et al. (April 1995), the disclosure of which is incorporated herein by reference, two-dimensional imaged metal articles are fabricated on webs by electroless deposition of catalytic inks using rotogravure. It is claimed that by layering thin lines of differing materials, numerous electronic components can be printed, including diodes, resistors, capacitors, batteries, sensors and fuel cells. The invention seems to arise from an especially low viscosity (about 20-600 centipoises) class of catalytic inks having less than about 10% solids, and which are said to permit printing of lines down to lateral (width) resolutions of 25 microns. Morgan also refers to the use of inks containing reducible metal ions in conjunction with a reducing agent such as acetate and ammonium.
The teachings of Morgan suffer from several drawbacks. First, the inks contemplated by Morgan all contain a polymer that remains as an impurity in the deposited material. Such impurities adversely affect conductivity and other function. A previous patent, U.S. Pat. No. 4,921,623 to Yamaguchi et al. (May 1990) highlights this distinction, by referring to the deposit formed from a solution containing a copper powder, a binder, and other ingredients as a “copper-type” coating, rather than a “copper” coating.
Another drawback is that even though Morgan reduces the particulate-containing portion of the ink to less than 10 wt %, Morgan still requires a substantial particulate content. As described above with respect to the Lowe patent, particulate content adversely affects function. Morgan does suggest possible substitution of metal particulate with metal salts in the inks, but even there Morgan teaches that the metal ions in the salts would be reduced the metal form prior to deposition.
Another drawback is that Morgan's conception appears to be limited to plate-based printing techniques that can print at high speeds of several hundred meters per minute. Plates for such presses can be difficult and expensive to manufacture, and even minor variations in a design may require costly redesign.
U.S. Pat. No. 5,758,575 to Isen (June 1998), the disclosure of which is incorporated herein by reference, teaches the continuous printing of inks using a rotogravure process implemented in multiple stations. Thick films are contemplated, and by drying each layer of ink before laying down a subsequent layer, multiple inks can be employed. Isen specifically refers to numerous specialty inks, including thermochromic inks, electrophosphorescent liquids, magnetochromic liquids, electrochromic liquids, and electroluminescent liquids, and teaches the use of spike rollers to make electrical contact between separated layers. Isen claims that his process can be used to produce electrical electronic components, including switches, diodes, capacitors, transistors, resistors, inductors, coils, batteries, and sensors.
Processes according to Isen suffer from several drawbacks. As with Lowe and Morgan, Isen still teaches deposition of a carrier (liquid, ink, etc) containing discreet particles. The existence of such particles adversely affects conductivity and other function.
In addition, the carrier for Isen's particulates is itself problematic. Suitable carriers are taught to include a quick drying solvent, which creates manufacturing and environmental difficulties. In addition, Isen teaches that suitable carriers include oligomeric materials, which may leave undesirable residues (see general discussion of particular containing inks in U.S. Pat. No. 4,666,735 to Hoover et al, May 1987).
Isen's process is particularly poor in handling instances where a deposited material is intended to function as a liquid. For example, in his description of electrolytes for use in forming batteries, Isen teaches deposition of micro-encapsulated electrolytes, presumably because simple deposition of a liquid electrolyte would lead to evaporation of the electrolyte itself. Isen does not contemplate printing a layer on top of a previously deposited liquid if layer.
Isen also appears to be limited to plate-based printing techniques such as rotogravure, flexographic, offset gravure, and letter press, and such plates can be costly to manufacture and modify. Isen suggests, for example, that such plates can be manufactured using such expensive and difficult techniques as diamond-stylus engraving, chemical etching, and laser inscribing.
Still another set of difficulties with the Isen process is that vias (throughs or interconnects) between layers are contemplated to be produced by spikes penetrating two or more layers. Spiking may be an inaccurate method of producing inter-layer connections, and may be especially problematic in interconnecting layers separated by more than one or two intervening layers.
Lamination
It is also known to provide electronic components in which a portion of the component is printed using conventional high speed printing equipment, and the remainder of the device is produced using some other technique such as lamination. A good example is U.S. Pat. No. 5,603,157 to Lake et al. (February 1997). In Lake, an insulative retaining ring for a button battery is printed on a substrate, cured, and then filled with a liquid electrolyte. Cathode and anode foils are laminated onto opposite sides of the ring, and the laminated product is then cut away to form the battery. In this manner, the battery is partially but not fully printed, and in any event the process is incapable of printing both circuit traces and electronic components. Other combination printing/laminate methods of producing batteries are described in U.S. Pat. No. 5,350,645 to Lake (September 1994) and U.S. Pat. No. 5,055,968 to Nishi (October 1991), and U.S. Pat. No. 4,621,035 to Bruder (November 1986).
Thus, there remains a need to provide new techniques for printing of both circuits and multilayered electronic components, especially for use with ink-jet and high-speed printing apparatus.